When digging a tunnel or mine, because of the complicated geological conditions, a geological disaster, such as underground water inrush, happens from time to time. This brings huge a catastrophe and economic lost to construction. The precise detection of regions containing water stratums around an underground engineering such as a tunnel can provide significant information for reducing water inrush accidents caused by unknown water stratums.
Presently, the tunnel water inrush forecasting technology mainly include below.
Geological observation method: according to geological investigation results of the inside and outside of a tunnel, the possibility of a water stratum in front of a tunnel face is analyzed and determined. This method depends on experiences and knowledge of the engineering geological conditions of a tunnel site of forecasters.
Ground hole method: which is a direct detection method and advantageous on fissured water of bedrock, however, there is a danger of hole over-flowing to cavern water and surge water of fault fractured zone with a good transmissivity which contacts surface water.
Infrared water detecting method: which uses infrared radiation field abnormalities caused by underground water stratums to detect a water stratum. This method can determine whether surrounding rock mass contains a water stratum, but cannot ascertain the volume of the water stratum; in addition, this method is influenced by tunnel construction seriously.
Full-wave apparent resistivity method: in which the resistivity difference between a water stratum and a rock mass is used to judge whether a water stratum exits and the outline of the water stratum, but this method is difficult to distinguish silt whose apparent resistivity parameter has no significant difference from water.
Transient electromagnetic method: in which a bunch of pulse electromagnetic field (which is called as a primary field) is emitted in front of a working face of a tunnel, and a receiving probe is used to measure vortex flow changes induced by a target body during a gap of the primary field. The changes depend largely on the resistivity and polarizability of the target body. This method cannot distinguish the geological body (such as sludge) whose resistivity and polarizability parameters have no obvious difference from water.
Seismic wave method: in which a seismic detection equipment is used to stimulate artificial seismic waves, and the waves' signal spreads through a rock mass around a tunnel, and when meeting a differentia interface with characteristic impedance of rock (e.g. the location in which the rock's strength changes, stratum lay and joint plane), especially a fault fractured zone interface and a bad geological interface such as karst cave, underground river, karst and silt, a portion of the signal is reflected back and a portion of the signal transmits into a front medium, and the reflected signal will be received by a detector of the detection equipment. Property of the geologic body in front of the tunnel's working face (e.g. soft rock zone, fractured zone, fault and water-bearing rock layer, etc.) can be known by software processing basing on the returned signal's delay time, strength and direction, and thereby a possible water-bearing structure may be deduced. So this method is based on a calculated geological structure to predict the water-bearing structure, but it can't decide whether it is water-bearing or not.
Acoustic sounding method: in which a rock is cut by a tunneler to stimulate an acoustic signal, and a synchronizing signal detector detects the reflected signal which is in front of a working face and reflected by the rock. Through analysis and calculation, the front geological structure is reduced and a possible water-bearing structure is estimated. It is an indirect method as well, and cannot locate an underground water stratum directly.
Temperature differential method: in general, with the increasing depth, the temperature of a rock body underground is higher and higher. But an underground water-bearing body and its flow in the rock will reduce (for conventional water stratums) or increase (for underground hot water stratums) the temperature of the rock body around the water stratum in certain scope. Making use of this phenomenon, it might predict a possible front water stratum by measuring the temperature variation in a tunnel. However, it can't draw accurately the significant parameters such as water content size of the potential water stratum.
Most of these methods judge whether there is a water stratum by investigating a water-bearing structure and its horizon. They are indirect measuring methods and cannot draw accurately the significant parameters of the water volume size of the potential water stratum. On the contrary, the underground water detecting method based on a magnetic resonance is a direct method.
“The surface magnetic resonance theories and methods” (2000.8, Wuhan, China university of geosciences press, ISBN 7-5625-1551-4) by Yulin Pan and Changda Zhang introduces an equipment for detecting water stratums based on magnetic resonance in France, which comprises an emission system, a receiving signal system, a microprocessor control and a register system and so on. The most beneficial effect is to detect the underground water within 150 meters depth directly.
Master dissertation of Yanqiu Jiang of Jilin University in 2006 “The Transmitter Development of SMRS Instrument for Groundwater Investigation” introduces each circuit design of a transmitter in a NUMIS. Master dissertation of Dongxu Gao of Jilin University in 2008 “Implementation of Weak SMRS Signal Amplifier for Groundwater Investigation” introduces a design of a signal modulating circuit. Master dissertation of Chuandong Jiang of Jilin University in 2009 “Design and Application of Data Processing Software in Magnetic Resonance Sounding System for Groundwater Detection” introduces one type of such method which gets underground water-bearing layer's thickness, moisture content and the distance from water to a transceiving coil, estimates permeability, conductivity and surge water amount according to the parameter of MRS underground water detecting signal, such as the stimulated emission current, emission duration, initial amplitude and relaxation time of the received MRS signal. 2010SR017733 computer software copyright registers “JLMRS data processing software”, the computer software accomplishes calculation of hydrogeology parameters, including distance between an aquifer and a transceiving coil, thickness of water layer, moisture content, permeability, conductivity and the surge water amount, in magnetic resonance detection.
Theoretically, a magnetic resonance signal may be generated as long as water exists. But whether the magnetic resonance signal could be detected or not depends on the sensitivity of the detecting equipment. The bigger the water stratum is and the nearer the distance is, the stronger the magnetic resonance signal is, and thus the signal is easier to be detected, while such potential big water stratum may be more harmful to a tunneling progress. Based on the sensitivity achieved at present, such water stratums which affect the engineering as above could be detected.
Water stratums in a geological region may exist not only in the direction along which the tunnel advances, but also in two lateral sides, roof and floor of the tunnel, which causes a significant potential trouble to safety.